PEARSON ELECTRONICS, INC. - Alrad Instruments Ltd.

Application Notes
For Current Monitors
Manufactured By
PEARSON ELECTRONICS, INC.
History
Pearson Electronics has produced precision
current-monitoring transformers since 1958. The
Pearson design plus careful workmanship and
quality control produce current monitors with
excellent frequency response and amplitude
accuracy. Originally developed for measuring
pulse currents, Pearson Current Monitors are
now also widely used to measure more complicated
transients and periodic signals from a few
hertz to well into the megahertz region.
Operation
To use a Pearson Current Monitor one needs
an oscilloscope and an appropriate length of
coaxial cable, which would usually have a
50 ohm impedance. For RF work an RF voltmeter
may be used. The output receptacle of
the current monitor is connected via the coaxial
cable to the high-impedance oscilloscope or
voltmeter input. The conductor carrying the
current to be measured is passed through the
hole in the current monitor. The voltage waveshape
as displayed on the oscilloscope will then
be a faithful reproduction of the actual current
waveshape within the limitations of rise time and
droop specified for the particular model used.
The voltage amplitude will be related, on a linear
basis, to the current amplitude by the sensitivity
in volts-per-ampere.
Performance Features
The standard accuracy for Pearson Current
Monitors is within +1%, -0% of the nominal
sensitivity. This accuracy applies to the midband
response. Exceptions due to droop and monitor
rise-time (low and high frequency cut-off) are
particular to each model, and are treated
separately in the specification sheet. Rise time is
short, ranging between 2 and 100 nanoseconds
(10-90% levels) in most cases. Droop values
range from 0.1% per microsecond to 0.5% per
millisecond for typical models. A significant
advantage, of course, is the fact that the current
monitor is physically isolated from the circuit
under test.
This feature is invaluable for eliminating ground
currents which usually occur when using current-viewing
resistors. Another advantage is that
low sensitivity can be used without suffering
from the ringing commonly encountered with
viewing resistors.
Typical Applications
Pearson Current Monitors can be used for
measuring and monitoring:
• Current waveshape and amplitude in high and
low voltage circuits, from microamperes to
megamperes.
• Circuits where the use of viewing resistors is
unsuitable because of ground-loop noise,
insertion resistance, or a lack of high voltage
isolation.
• Pulse currents at high voltage, as associated
with microwave or x-ray tube modulators,
particle accelerators and lasers.
• Current transients and harmonics in power
systems.
• Lightning-strike currents.
• Pulsed charged-particle beam current.
• Current in electrolytes and plasmas.
• EMI currents.
• Video and RF currents.
• Currents in spot and induction welders.
• Antenna phasing.
• Flash-tube current.
Calibration
Pearson Current Monitors intended for general
use are adjusted at the factory to yield an initialpulse-amplitude
error within specification (either
±1% or +1, -0% depending on model), when
terminated in a high impedance load such as an
oscilloscope. The equivalent circuit of nearly
every Pearson Current Monitor is that of a
voltage generator in series with 50 ohms.
Alrad Electronics, Turnpike Road Ind Estate, Newbury, Berkshire RG14 2NS Tel. 01635 30345• FAX 01635 32630• Web: www.alrad.co.uk

Specifications
A review of certain Pearson Current Monitor
specifications may assist the engineer in choosing
the right model for a particular application.
• Sensitivity The considerations here involve the
peak current to be measured, the oscilloscope
sensitivity, and trade-offs imposed by other
specifications.
For Pulse Applications
The Time Domain Parameters Are:
• Maximum Peak Current This value is based
primarily on the voltage-breakdown rating of the
connector used. For instance, a 500-volt rating
on the connector gives a 5000-ampere peak
current rating for a 0.1 volt-per-ampere current
monitor.
• Droop The value listed is the maximum
amount to be expected at current levels above a
few amperes. At low current levels, low initial
core permeability may cause higher droop
values and a corresponding increase in the lowfrequency
-3 dB point for some models.
• Usable Rise Time If the 10 to 90 percent rise
time is greater than the specified usable rise
time, initial overshoot and ringing will be less
than 10% of the pulse step amplitude.
• I . t Max This parameter is analogous to the
voltage-times-time constant of a pulse transformer.
The product of current times time for a
rectangular pulse must not exceed the value
listed or the core will saturate, causing a distorted
waveform. If two turns are used through
the window to obtain twice as much sensitivity,
the I . t rating necessarily will be halved.
For The Continuous Signal Applications,
The Frequency Domain Parameters Are:
• Maximum RMS Current This value is based
on heating considerations involving the longterm
stability of the internal resistance element
in the current monitor.
• Approximate Low And High Frequency 3 dB
Points Due to the ac nature of transformers, the
flat midband response will roll off at low frequency.
The “corner” or “cut-off” frequency, at
which the response is 3 dB down, is specified.
Internal resonances determine the useful high
frequency cut-off point. Response is within
± 3 dB at the specified high frequency limit.
• I/f Max This parameter is to sine-wave
currents what the I . t product is to rectangularwave
currents. The quotient of peak current
divided by frequency must not exceed the listed
value or the core will saturate. If two turns are
used through the window to obtain twice as
much sensitivity, the I/f rating necessarily will be
halved.
External Termination
When viewing pulse rise times below 100
nanoseconds, RF currents above a few megahertz,
or when using long cables, it is advisable
to terminate the general-use current monitor
output cable at the oscilloscope with 50 ohms to
prevent standing waves or cable fill-time effects.
With this termination the output voltage will be
approximately half the unterminated value,
subject to the accuracy of the termination
resistance, and the attenuation of the cable.
Fast-rising pulses can produce spurious observed
ringing due to high frequency current
flowing on the outside of the cable shield. This
current can be suppressed by increasing the
inductance of the shield run by threading the
cable through one or more magnetic cores.
Good results have been obtained using three
turns through four ferrite cores of about one inch
inside diameter, two inch outside diameter and
1
⁄2 inch thickness.
With termination, models 2877, 4100, 2100,
3100, 150, 410, 411, 110, 110A, and 1010 will
exhibit lower droop and increased I . t product.
Performance Trade-Offs
In general, as sensitivity decreases, peak
current, rms current, and I . t product increase.
In addition, it is difficult to combine low droop
with high sensitivity.
Electrostatic Shielding
All Pearson Current Monitors which have the
3 1 ⁄2 inch or 10 3 ⁄4 inch diameter hole are double
shielded. The outer shield (case) is automatically
grounded when the threaded mounting holes are
used with metal screws through a grounded
bracket. All other standard Pearson Current
Monitors are single shielded. The shield and the
coaxial cable braid are grounded by the mounting
screws in all models except those having a
2-inch diameter hole. These have insulated
mounting brackets and the only grounding is
through the cable braid to the oscilloscope.
Biasing
All Pearson Current Monitors use ferromagnetic
cores which can become saturated by the dc
component (average value) of the current (I dc),
or by the current-time product (I . t) of the pulses.
Since the output of the monitor is sustained by the
changing flux level in the core, magnetic saturation
will degrade performance. As a function of
increasing I dc , the effective permeability of the
core decreases, causing the droop rate and lowfrequency
cut-off point to increase. Also, the
available flux swing is decreased, reducing the
maximum viewable I . t. When viewing a pulse,
the output voltage will drop to zero when the
Alrad Electronics, Turnpike Road Ind Estate, Newbury, Berkshire RG14 2NS Tel. 01635 30345• FAX 01635 32630• Web: www.alrad.co.uk

integrated value of current with respect to time
causes the flux level to reach saturation. The
monitor will recover after the applied current
returns to zero and the flux returns to its remanent
value.
Biasing can enable operation with a larger dc
component of current, and/or improve the
maximum viewable current-time product. The
objective of biasing is to reset the flux in the core
to a value near the negative saturation level so
that the maximum flux swing is available.
The I . t value given in the specification sheets is
based on a flux swing from zero to saturation for
all models. However, models indicated in the
specification sheet with ** have high-permeability
core material which has a residual induction of
0.6 to 0.8 of the saturation value. These models
will need bias to obtain more than 0.2 to 0.4 of
the rated I . t. With bias all models can achieve
nearly twice the specified I . t, since the flux can
be reset close to negative saturation.
For pulses with small I . t, the droop rate will
increase as I dc approaches “I dc max ” in the table.
I dc max is the approximate level at which the droop
rate will be doubled. In this situation, bias is
used to cancel I dc and allow normal operation.
I dc max
I bias
Model (A) (A) Ratio
2877 0.17 0.13 50
4100 0.32 0.25 50
2100 0.78 0.59 50
3100 1.9 1.4 50
150 0.78 0.59 99
325 25 142
2878 0.17 0.13 45
410 10 424
411 0.32 0.25 424
110 0.78 0.59 414
110A 0.78 0.59 414
310 25 352
1010 160 120 490
1025 8 361
3025 25 513
2879 0.17 0.13 46
101 0.78 0.59 325
301X 60 312
1080 60 173
1330 60 345
1423 60 80
2093 150 473
Bias current may be applied via an additional
primary wire through the current monitor hole
and should be of the opposite polarity to the
expected signal. The correct value for the bias
current is I dc , plus, for ** models, “I bias ” from the
table. The bias current source must have
enough resistance to avoid the effect of a
shorted turn through the core.
Bias current may also be injected into the secondary
winding via a “T” adapter connected to
the output connector. The load at the instrument
end of the cable should be high enough so that
no significant bias current is diverted from the
monitor. The source resistance of the bias current
source must not load the output of the current
monitor. For models with 50 ohm output resistance,
a bias source resistance of 5000 ohms will
result in a -1% shift in sensitivity. The amount of
current to be applied with this method is that
which was calculated above divided by “Ratio”
from the table.
In a pure ac signal, I dc is zero and the I . t of the
positive and negative parts of the cycle are
equal. The “I/f” value from the specification
sheet gives the maximum amplitude for a sine
wave of frequency f. Biasing cannot improve
performance for ac signals.
Transient Limitation
When monitoring high current transients it is
possible to exceed the internal dissipation limits
of the unit. For unidirectional currents saturation
of the core protects the secondary circuit. The
energy of a bidirectional transient is not limited
in this way. The heat generated by such a
transient current is proportional to the time
integral of the square of the current. For any
given model the limit can be found by multiplying
the peak current rating by the I . t product.
Exceeding this I 2 t value may damage the unit.
For example, the I 2 t limit for the 301X would
be 50 kA times 22 A-sec, or 1.1 x 10 6 A 2 -sec.
If an ac current of 5 kA RMS were applied, the
maximum duration should be 1.1 x 10 6 ⁄(5 x 10 3 ) 2
= 4.4 x 10 -2 sec.
Phase Shift
Within their specified frequency ranges, the
phase shift between the voltage output and
the current being measured by Pearson Current
Monitors is small. For frequencies at least one
decade from the low and high frequency cutoff
points, the phase shift is usually less than
6 degrees, and the amplitude error is less than 1%.
Alrad Electronics, Turnpike Road Ind Estate, Newbury, Berkshire RG14 2NS Tel. 01635 30345• FAX 01635 32630• Web: www.alrad.co.uk

High-Voltage Consideration
Pearson Current Monitors intended for highvoltage
use have large hole diameters and
radiused edges to keep voltage gradients low.
The two standard case sizes are those with
3 1 ⁄2 inch and 10 3 ⁄4 inch inside diameters. These
sizes are double shielded, as mentioned previously.
The extra shield affords excellent protection
from electric field penetration. Since the
mounting screws ground the outer shield, a
direct path to ground is provided for possible
arcs from the high-voltage conductor to the
case. This direct path to ground minimizes the
danger of high voltage entering the instrumentation
circuit. The inner shield and coaxial connector
are “floating” and hence can be grounded at
the oscilloscope, at the oil tank bulkhead, or
wherever convenient to minimize ground loops
and promote safety.
A round conductor at high potential centered in
the hole of a Pearson Current Monitor results in
coaxial geometry. For this arrangement, the
minimum voltage gradient at the surface of the
conductor occurs when the ratio of conductor
radius to hole radius is 1/e. Departure in either
direction from the optimum conductor radius will
increase the gradient and lower the permissible
operating voltage. Voltages and recommended
conductor sizes are tabulated for three Pearson
case sizes. Cleanliness of the local environment
is necessary to achieve reliable high-voltage
operation.
Typical Pulse Flashover
Hole Conductor Voltage Voltage
Diameter Diameter *in Oil in Air
2” 3
⁄4” 150kV 30kV
3 1 ⁄2” 1 1 ⁄4” 300kV 50kV
10 3 ⁄4” 4” 900kV 150kV
*Note: For pulses of under five microseconds duration.
All Pearson Current Monitors are constructed of
materials compatible with transformer oil. For
detailed information regarding the care and
handling of transformer oil, use of pulse transformers
in transformer oil, etc., refer to Pearson
“Notes on High-Voltage Pulse-Transformer
Insulating Oil Care and Accidental Transformer
Overvoltages” (available on request from the
Pearson factory).
When the conductor in the hole of a Pearson
Current Monitor operates at a high direct-current
potential in transformer oil, it is recommended
that a barrier composed of solid insulating
material be placed between the conductor and
the current monitors to prevent arcs due to
foreign particles “lining up” in the dc field.
Typical barrier materials include teflon tubing,
laminated paper-phenolic tubing and oil-impregnated
(by vacuum) Kraft paper. If the barrier
material is used as a centering device, touching
both conductor and current monitor, adequate
creep distance should be allowed over the
shortest surface path to prevent breakdown.
At voltage levels of a few kilovolts in air, ordinary
high-voltage insulated wire will suffice for the
center conductor. At levels above 10 kilovolts in
air, it is advisable to use a centered conductor of
large diameter. As the voltage level approaches
the rated voltage for the hole size, it becomes
more necessary to use the optimum conductor
size.
Insertion Resistance
A typical Pearson Current Monitor contains a
distributed load resistance. When using such a
current transformer this resistance appears as a
very small equivalent series resistance in the
circuit being measured. This resistance is
usually so small it can be neglected. For those
cases when it needs to be considered,
the values are tabulated here.
Model Nos. Insertion Resistance (Ohms)
2877, 4100, 2100, 3100 0.02
150 0.005
325